The present invention relates to voltage regulators, and more particularly, to a voltage regulator equipped with a device for limiting an output current therefrom.
A voltage regulator is a four-pole electrical device interposed between a source of electrical power and an electrical circuit. The electrical circuit is called a load circuit, and is supplied by the electrical power source. The type of electrical power source may vary, for example, between a chemical cell that is non-rechargeable and a battery that is rechargeable. Power sources do not necessarily deliver a constant output voltage. The output voltage may depend, for example, on a state of depletion of the power source, especially in the case of a chemical cell, or on the current charge of a battery.
The load circuit may be an electronic circuit intended for any application, such as a mobile radio communications terminal powered by its own battery. A voltage regulator is used when the load circuit requires a constant supply voltage for its operation, although the power source delivers a voltage that is variable along its duration of use. The function of the voltage regulator is to receive an input voltage that is variable, and to deliver a power-supply voltage that is substantially constant.
The voltage regulator is a low-drop-out (LDO) type if it operates even when the voltage difference between the power source and the nominal power-supply voltage becomes significantly reduced. In general, a stabilization capacitor is placed in parallel with the load circuit at the output of the voltage regulator. When a voltage is applied to the load circuit, a transient condition then occurs, during which the current delivered by the voltage regulator momentarily exhibits a very high level that is very capable of damaging the power source. It is then necessary to make provisions for limiting the electrical current levels delivered by the voltage regulator. This limitation also prevents damage which might result from an accidental short-circuit occurring in the load circuit, or from a high leakage current in the stabilization capacitor.
Devices exist for limiting the current delivered by a voltage regulator, and especially devices incorporated into the regulator itself. These devices are also called short-circuit protection circuits. In the particular case of LDO regulators which deliver an electrical power controlled by a power transistor contained in these regulators, one short-circuit protection method includes reproducing, in a circuit branch added to the regulator, the level of the electrical current delivered to the load circuit. This reproduced current level is obtained by using an additional transistor which, to within a scale factor, recopies the level of the current delivered by the power transistor to the load circuit.
The short-circuit protection is achieved by a limitation of the current level delivered by the power transistor when the recopied current level in the added circuit branch becomes greater than a previously fixed threshold. This threshold is chosen to prevent any damage that too high a current level delivered to the load circuit might cause.
FIG. 1 is an electrical diagram of a linear voltage regulator 1 according to the prior art. The electrical power delivered to the load circuit 20 is controlled by a power transistor 2. This transistor 2 may be a p-channel metal-oxide-semiconductor (PMOS) transistor, for example. The voltage regulator 1 receives a power supply voltage from an electrical power source 10, the positive terminal E of which is linked to the source of the power transistor 2. The drain of this transistor 2 is linked to the output terminal S of the regulator 1. The other output terminal of the regulator 1 is linked to ground. The power source 10 and the load circuit 20 are also linked to ground.
The transistor 2 is controlled on its gate by the output of an operational amplifier 11, which will now be referred to as an error amplifier. The error amplifier 11 is slaved by a feedback path starting from an intermediate node of a voltage-divider bridge 12 linked to the non-inverting input of this amplifier. The inverting input of the error amplifier 11 receives a reference voltage Uref that is fixed with respect to ground. The reference voltage Uref may be produced, for example, by a voltage source exploiting the forbidden band of a semiconductor material.
The voltage divider 12 is arranged in parallel with the load circuit 20 within the voltage regulator 1. The voltage divider 12 includes, for example, two resistors connected in series. One resistor 12a is connected between the terminal S and the feedback node, and the other resistor 12b is connected between the feedback node and ground. The respective values Ra, Rb of these two resistors 12a, 12b are chosen as a function of the reference voltage Uref and of the desired power-supply voltage U, given that U=Uref*[(Ra+Rb)/Rb].
A stabilization capacitor 21 is placed in parallel with the load circuit 20, such as across its input. The capacitance of this stabilization capacitor 21 is 1 microfarad, for example.
The short-circuit protection circuit 100 of the voltage regulator 1 according to FIG. 1 includes a recopy transistor 13 operating under conditions similar to those of the power transistor 2. The illustrated recopy transistor 13 is also a PMOS transistor. The respective gates of these two transistors 2, 13 are linked together, as are their sources which are connected to the positive terminal E of the electrical-power source 10.
Under these conditions, the current level flowing in the transistor 13 recopies the current flowing in the power transistor 2. The level of the recopied current is compared using a transistor 14 with a fixed reference current If. The fixed reference current If is produced by a current generator 15 placed between the drain of the recopy transistor 13 and ground. The transistor 14 is, for example, an n-channel metal-oxide-semiconductor (NMOS) transistor. The gate of the transistor 14 is connected between the drain of the recopy transistor 13 and the current generator 15. The source of the transistor 14 is linked to ground, and its drain is linked to the positive terminal of the power source 10 via a resistor 17.
Another PMOS transistor 16 has its gate linked to the drain of the NMOS transistor 14, and its channel is connected between the positive terminal E of the power source 10 and the output of the amplifier 11. This transistor 16 causes the voltage between ground and the gate of the power transistor 2 to rise when the level of the current recopied by the transistor 13 becomes greater than the level of the reference current If. Thus, the current level delivered to the load circuit 20 and which is controlled by the power transistor 2 is limited.
One drawback of this layout lies in the fact that the electrical power corresponding to the recopied current level is dissipated within the voltage regulator itself, i.e., in the current generator 15. This corresponds to electrical power delivered by the power source 10 that is lost with regards to the power supply for the load circuit 20.
In the case of a battery, a voltage regulator and a load circuit that are integrated into a self-contained electrical device, such as a mobile radio communications terminal, for example, lost electrical power associated therewith reduces the endurance of the device. This reduction in the endurance represents an important drawback for the use of this type of short-circuit protection regulator.
A partial approach for recopying the current includes using a transistor 13 such that the scale factor for recopying the current level flowing in the power transistor 2 is small, or even very small. However, such an approach made in terms of the choice of the physical dimensions of the transistors 2 and 13 is a constraint that is difficult to adapt to the selection of the type of recopy transistor 13 that can be correctly matched with the power transistor 2 in order to recopy the current level controlled by the latter.
In view of the foregoing background, an object of the present invention is to provide a voltage regulator that limits the output current therefrom with a slight impact on its electrical power consumption.
The present invention advantageously relates to a voltage regulator that includes an output resistor for stabilizing operation of the regulator. Voltage regulators may exhibit electrical characteristics leading to unstable conditions. The unstable conditions may be eliminated by the addition of a resistor at the output of the regulator. Such a resistor, called a stabilization resistor, has a low value, such as on the order of a few tenths of an ohm.
This and other objects, advantages and features in accordance with the present invention are provided by a voltage regulator comprising a power transistor having a control input, a first electrode linked to a first voltage supply terminal, and a second electrode linked to an output of the regulator by way of a stabilization resistor. An error amplifier has a first input for receiving a reference voltage with respect to a second voltage supply terminal, a second input for receiving a feedback voltage representative of the voltage on the second electrode of the power transistor with respect to the second supply terminal, and an output linked to the control input of the power transistor.
A protection circuit is preferably linked to the control input of the power transistor and to the voltage-supply terminals to limit the current delivered at the output of the regulator. The protection circuit may comprise voltage-adjustment means acting on the control input of the power transistor as a function of the voltage of the terminals of the stabilization resistor.
A voltage regulator according to the invention therefore does not include a circuit branch added to recopy the current level flowing in the power transistor. Therefore, the power dissipated within the regulator corresponding to recopying the current level is avoided. Furthermore, the short-circuit protection of the invention does not require an additional resistor with respect to the stabilization resistor placed at the output of the regulator. This further contributes to reducing the electrical power dissipated within the voltage regulator.
One particular embodiment of the protection circuit comprises means for delivering a fixed reference voltage with respect to the output of the regulator, and the voltage-adjustment means comprise a comparator which compares the voltage at the terminals of the stabilization resistor with the fixed reference voltage. The voltage-adjustment means may further comprise an adjustment transistor having a first electrode linked to the first voltage supply terminal, a second electrode linked to the control input of the power transistor, and a control input linked to an output of the comparator.
The adjustment transistor acts on the control input of the power transistor as a function of the state of the output of the comparator. This comparator is connected at one of its inputs so that when the voltage at the terminals of the stabilization resistor is higher than the fixed reference voltage, the adjustment transistor enters a conducting state. This causes the level of the current delivered to the load circuit to be regulated.